(1) Field of the Invention
The present invention relates to a crystal silicon film forming method and to a thin-film transistor and display device which use the crystal silicon film.
(2) Description of the Related Art
For example, there are thin-film transistors (TFTs) included in liquid-crystal panels or organic electroluminescence (EL) panels. A channel portion of a TFT is formed from a-Si which is amorphous silicon or poly-Si which is polysilicon. The poly-Si of the channel portion of the TFT is typically formed by forming a-Si and subsequently irradiating the a-Si with a laser light such as excimer and the like to instantaneously raise the temperature of the a-Si to thereby crystallize the a-Si.
Furthermore, as TFT structures, there is the bottom-gate structure in which the gate metal is disposed on the substrate-side as seen from the x-Si (“x” is either “a” or “poly”) of the channel portion, and the top-gate structure in which the gate metal and source/drain metal are arranged in the opposite direction as the substrate as seen from the x-Si of the channel region. The bottom-gate structure is mainly used in an a-Si TFT which has a channel portion formed from a-Si, and the top-gate structure is mainly used in a poly-Si TFT which has a channel structure formed from poly-Si.
In addition, there are also cases where a poly-Si TFT has the bottom-gate structure, and this has the advantage of keeping down fabrication cost. In such a poly-Si TFT having the bottom-gate structure, poly-Si is formed by performing crystallization by irradiating a-Si with a laser.
However, the gate metals, for example, a gate insulating film of SiO2 film and an a-Si film, are formed using methods such as sputtering and CVD, and perfectly uniform forming thereof is not possible, apparatus-wise. Specifically, the gate metals, namely, the gate insulating film and the a-Si film, included in the poly-Si TFT are formed with a certain degree of unevenness in film thickness. As such, when crystallization is performed by irradiating the a-Si film formed in the above manner with a laser, unevenness in the film quality of the poly-Si TFT film is generated, and defects arise in a display device such as a liquid crystal panel or an organic EL panel configured using the poly-Si TFT.
In response to this, there is disclosed a technique of suppressing the variance in film quality of a poly-Si film occurring when an a-Si film formed using a method such as sputtering or CVD is irradiated with a laser for crystallization into a poly-Si film (for example, Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2001-297984).
In Patent Reference 1, the a-Si film is formed within an optimal film thickness range to thereby suppress the variance in film quality of a poly-Si TFT film with respect to the unevenness of the a-Si film thickness. Here, the light source of the laser used is typically a XeCl excimer laser, and argon lasers and KrF are also included.
Furthermore, Patent Reference 1 discloses a poly-Si TFT, and, when an a-Si film is irradiated with the laser so as to be crystallized to form a poly-Si film, a metal gate electrode is present in a layer lower than the a-Si film, with an insulating film disposed therebetween.